1. Field of the Invention
The invention relates generally to optical switches. In particular, the invention relates to optical switches used in multi-channel optical communications networks and having controlled transmissivity for different channels.
2. Background Art
Modern communications networks are increasingly based on silica optical fiber, which offers very wide bandwidth including several transmission bands usable for communications. In a conventional point-to-point optical communications link, at the transmitter, an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1550 nm band, and the modulated optical signal is impressed on one end of the silica optical fiber. Other transmission bands at 850 nm and 1310 nm are also available. On very long links, the optical signal may be amplified along the route by one or more optically pumped erbium-doped fiber amplifiers (EDFAs) or other optical amplifiers. At the receiver, the optical signal from the fiber is detected, for example, by an optical p-i-n diode detector outputting an electrical signal in correspondence to the modulating electrical signal. The transmission bandwidth of such systems is typically limited by the speed of the electronics and opto-electronics included in the transmitter and receiver. Speeds of 10 gigabits per second (Gbs) are available in fielded systems, and 40 Gbs systems are reaching production. Further increases in the speed of the electronics will be difficult. These speeds do not match the bandwidth inherent in the fiber, which is well in excess of one terabit per second. Furthermore, such fast optical transmitters and receivers are expensive and may require special environmental controls.
Transmission capacity of fiber systems can be greatly increased by wavelength division multiplexing (WDM) in which the optical signal is generated in a transmitter including multiple semiconductor lasers emitting at different respective wavelengths within the transmission band. The 1550 nm transmission band has a bandwidth of about 35 nm, determined by the available amplification band of an EDFA. Other amplifier types and amplification bands are being commercialized so that the available WDM spectrum is growing each year. In a WDM system, each laser is modulated by a different electrical data signal, and the different laser outputs are optically combined (multiplexed) into a multi-wavelength optical signal which is impressed on the optical fiber and which together can be amplified by an EDFA without the need to demultiplex the optical signal. At the receiver, an optical demultiplexer, such as one based on a diffraction grating, an arrayed waveguide grating, or a thin-film filter array, spatially separates the different wavelength components, which are separately detected and output as respective electrical data signals. For an N wavelength WDM wavelength grid, the fiber capacity is increased by a factor of N using electronics of the same speed. Dense WDM (DWDM) systems are being designed in which the WDM comb includes 40, 80 or more wavelengths with wavelength spacings of under 1 nm. Current designs have wavelength spacings of between 0.4 and 0.8 nm, that is, frequency spacings of 50 to 100 GHz. Spectral packing schemes allow for higher or lower spacings, dictated by economics, bandwidth, and other factors.
Point-to-point WDM transmission systems as described above enable very high transmission capacity in networks having simple connectivity. However, a modem communications network 10, such as that illustrated in FIG. 1, tends to be more complex and requires that the WDM concept be expanded to cover not only transport but also switching in a complex communication network. This network 10 has multiple switching nodes 12 switching signals between multiple terminals 14 at the edges of the network 10. Fiber optic links 16 interconnect the switching nodes 12 and terminals 14. The switching nodes 12 should be capable of switching single-wavelength WDM channels in different directions with the directions being changeable over some time period. The network diagram of FIG. 1 is highly conceptual but emphasizes the switching requirement of the illustrated complexly connected network. Such a WDM network 10 achieves high capacity through multi-wavelength channel transport along complexly fiber-interconnected routes. To achieve a flexible dynamic interconnection within the network 10, it is desirable that the fiber infrastructure be wired differently for different wavelengths. As a result, the optical switches 12 should not only be dynamically reconfigurable between multiple ports but also the switch state should be able to be wired simultaneously, yet independently in each wavelength channel. That is, the cross-connects 12 should be wavelength selective. Channel paths 20 are illustrated in FIG. 1 in which three wavelength channels at wavelengths xcex1, xcex2, xcex3 can enter a switching node 12 on a single fiber 16 but be switched at its outputs to three different fibers 16. Also note that this architecture allows the reuse of wavelengths between different pairs of terminals 14, for example, the wavelength xcex1 as illustrated. Frequency reuse further increases the traffic capacity of the network 10. Such switching requirements apply as well to a more regularly configured ring network having switching nodes distributed around a fiber ring with a terminal associated with each node.
The practice in the recent past has been to form each link 16 in the network 10 as a separate point-to-point WDM system so that each switching node 12 includes an optical receiver and an optical transmitter for each wavelength channel. The electrical data signals derived from the optical receiver are spatially switched by conventional electronic switches and then converted back to optical form for transmission on the next link. However, a point-to-point design does not integrate well into a complex network such as in FIG. 1. The required number of optical receivers and transmitters become very expensive. Also, such a system requires the electronics and opto-electronics at each switching node to be operating at the highest data rate supported by the network. Such a system is unduly costly when some end-to-end links require only modest data rates, and the system is difficult to upgrade since all the nodes must be upgraded at the same time.
For these and other reasons, there is much interest in all-optical communication networks in which each switching node demultiplexes the multi-wavelength WDM signal from an input fiber into its wavelength components, spatially switches the separate single-wavelength beams in different directions, and multiplexes the switched optical signals for retransmission on one or more output fibers. Thus, a wavelength-routing node will generally switch WDM channels in an all-optical manner unless there is a specific need to electrically regenerate a specific subset of channels, for example, to remove accumulated optical noise through electrical regeneration or to perform wavelength conversion. Indeed, the goal is to reduce the number of conversions from optical to electrical and back to optical at each intervening node in a fiber optic end-to-end link.
An all-optical wavelength-selective switching node 12 may be implemented by a wavelength cross connect (WXC) such as a 3xc3x973 WXC 22, represented in the simple schematic diagram of FIG. 2, coupling three input ports 24 to three output ports 26, each port being typically equated with a transmission fiber in the network. The WXC 22 has the capability of switching any wavelength channel on any input port 24 to the corresponding wavelength channel on any output port 26. The design is not limited to a 3xc3x973 cross connect with three input and three output ports and may be generalized to a Wxc3x97W cross connect with W input ports and W output ports, where W is greater than 1. Systems under development have values of W up to 12, and further replication is possible. An unequal number of input and output ports is possible but will not be further discussed here.
Although other designs are possible, the WXC 22 is typically accomplished, as illustrated in the schematic diagram of FIG. 3, by splitting the WDM channels into their wavelength components and switching those wavelength components by optical elements generally insensitive to the wavelength values, often referred to as white-light elements. The multi-wavelength signal on each optical input port 24 enters a respective one of 3 (more generally W) optical demultiplexers 28 which separate the multi-wavelength signal into its N wavelength channels, here illustrated as N=4, and outputs single-wavelength switching beams 30. The single-wavelength switching beams 30 of the same nominal wavelength enter a respective one of N white-light cross connects 321 to 32N, which can switch any input switching beam 30 to any of its output switching beams 34 without regards to wavelength except that the switching beams 30, 34 in any plane have the same wavelength or color. W optical multiplexers 36 each receive N single-wavelength output switching beams of N different wavelengths and combines them into a multi-wavelength signal impressed on one of the W output ports 26. By such an arrangement, the N wavelength channels on each of the W input ports 24 are simultaneously and independently routed to selected ones of the W output ports 26. The wavelength multiplexers and demultiplexers may be accomplished in a number of ways typically including dispersive elements such as Bragg gratings, thin-film interference filter arrays, and arrayed waveguide gratings (AWGs) that spatially separate wavelength components.
There are a number of ways of achieving the wavelength routing functionality represented by the wavelength cross connect 22. The illustrated structure of FIG. 3 represents a replicative approach using N distinct white-light cross connects 321, to 32N. A more integrated design, schematically illustrated in FIG. 4, uses the same input and output ports, multiplexers 36, and demultiplexers 28, but uses fibers 42, 44 on the switch side of the demultiplexers 28 and multiplexers 36 to route the single-wavelength signals to a large white-light optical cross connect 46 having WN inputs and WN outputs. The optical cross connect 46 needs to be configured so that it connects inputs to outputs of the same nominal wavelength. Any cross-wavelength connection results in loss of that signal at the output multiplexers 36 and perhaps corruption of another signal.
Solgaard et al. in U.S. Pat. No. 6,097,859 (hereinafter referred to as Solgaard) disclose a multi-wavelength cross connect switch based on an array of micro electromechanical system (MEMS) mirrors. In this device, multi-wavelength WDM signals are received on typically two or more input ports. An input lens systems collimates these beams and directs them to a diffraction grating that reflects different wavelength channels at different angles. The resulting two-dimensional array of beams, in which input beams are separated in one dimension and the wavelength channels are separated in the other dimension, is imaged onto an array of electronically actuated MEMS micro-mirrors. Each beam is reflected by its micro-mirror at a selected angle that depends upon the voltage applied to the mirror actuator. Because switching is performed between corresponding wavelength channels of different fibers, in the simplest design the mirrors need to tilt only in a single dimension.
A more integrated design following the Solgaard design includes the wavelength multiplexing and MEMS switching in a single unit. A 2-input, 2-output, 7-wavelength switching system 50 is schematically illustrated in FIG. 5. Two input fiber waveguides 52, 54 and two output fiber waveguides 56, 58 are aligned linearly parallel to each other to couple into two free-space input beams 60, 62 and two free-space output beams 64, 66. A lens 68 collimates the input beams 60, 62 to both strike a diffraction grating 70.
Considering the first input beam 60, the diffraction grating 70 angularly disperses it into a fan-shaped collection 72 of beams angularly separated according to wavelength, as is well known in the art and taught by Solgaard. That is, the grating 70 acts as a wavelength-dispersive element. The wavelengths of the signals on the one input fiber 52, as well as on all the other fibers 54, 56, 58 correspond to the WDM wavelengths of one of the standardized grids, for example, the ITU grid, and each optical carrier signal of the different wavelengths on the separate fibers is modulated according to its own data signal. Each of the beams in the collection 72 of beams corresponds to one of the wavelength channels of the ITU grid. A lens 74 focuses these beams toward a first row 75 of tiltable input mirrors 76, typically formed as a two-dimensional array in the plane of a MEMS structure. The mirrors 76 of the first row 75 are associated with the wavelength channels of the first input fiber 52 while those in second row 77 are associated with the second input fiber 54. The mirrors 76 are also arranged in a second dimension in which each column 78 is associated with one of the wavelengths xcex1 through xcex7 for the illustrated 7-wavelength system. The mirrors 76 described to this point are input mirrors. Similarly arranged output mirrors 80 in rows 82, 84 are output mirrors. The mirrors 76, 80 are tiltable about respective axes lying generally horizontally in the illustration so that the input mirrors 76 direct each input beam 60, 62 beam toward a folding mirror 86. Depending upon the tilt angle of the respective input mirror 76, the folding mirror 84 reflects that beam to the output mirror 80 in a selected one of the output rows 82, 84. The two illustrated connections show coupling to output mirrors 80 located alternatively in the third and fourth rows 82, 84. The output mirrors of the third row 82 are associated with respective wavelength channels on the first output fiber 56 while those of the fourth row 84 are associated with the wavelength channels on the second output fiber 58. The optics are arranged and controlled such that an optical signal from an input mirror 76 is reflected only to one of the output mirrors 80 in the same column 78, that is, associated with the same WDM wavelength. The input and output mirrors 76, 80 typically have the same construction and differ only by their placement in a two-dimensional array in a single MEMS structure. Practically speaking, in this configuration, the designation of input and output mirrors is arbitraxy and the input and output rows may be interleaved.
In some applications, it is possible to dispense with the folding mirror 86 and to use only a single set of micromirrors to directly reflect a wavelength-separated input beam back to a selected output fiber although this configuration presents problems with uniformity of coupling.
Each output mirror 80 is also tiltable in correspondence to the tilt angle of the input mirror 76 to which it is coupled through the folding mirror 86 so that the same optics 68, 70, 74 used to focus and demultiplex the beams from the input fibers 52, 54 are also used to multiplex the wavelength-separated output beams onto the two output fibers 56, 58. That is, the diffraction grating 70 acts as both a demultiplexer on the input and a multiplexer on the output.
By means of the illustrated optics and MEMS micromirror array, a wavelength channel on either of the input fibers 52, 54 can be switched to the same wavelength channel on either of the output fibers 56, 58. It is of course understood that the described structure may be generalized to more input and output fibers and to more WDM wavelengths.
Another system 90, as illustrated in the schematic diagram of FIG. 6, provides much of the functionality of the system 50 of FIG. 5. Input fibers 92 are arranged in a first linear array 94 and output fibers 96 are arranged in a second linear array 96. The system includes a first two-dimensional array 100 of input mirrors 102 and a second two-dimensional array 106 of output mirrors 108. In both arrays 100, 106, the mirrors 102, 108 are arranged in row directions according to fiber and in column directions according to wavelength. The beams are directly coupled between the input and output mirrors 102, 108 without the use of a folding mirror. However, such a coupling mirror may be advantageously applied between the two mirror arrays 100, 106 and eliminate the need for separate gratings and further allow the input and output fibers 92-98 to be placed in a single linear array. Advantageously, separate demultiplexing and multiplexing gratings 110, 112 are provided on the input and output sides respectively, and birefringent wave plates are inserted so as to substantially eliminate polarization dependence within the switch, as is well understood by those in art.
A large white-light cross connect may have a structure similar to that illustrated in FIG. 6 but without the diffraction grating. A white-light system 120 illustrated in the schematic illustration of FIG. 7 includes a substantial number of input fibers 122 bundled together in a two-dimensional array 124 and preferably a like number of output fibers 126 bundled together in another two-dimensional array 128. One of the input fibers 122 concentrates its beam at one of the input mirrors 102 of the input mirror array 100. Similarly, the output mirror array 106 has its output mirrors 108 near the focus of the output fibers 126. An optional mirror 130 couples the input and output mirrors 102, 104, Each input mirror 102 is tiltable about two axes to allow it to direct its input beams to any ones of the output mirrors 106. Each output mirror 106 is similarly tilted in a complementary fashion to direct the beam towards the output fiber 126 associated with that output mirror 108.
Another unillustrated white-light system resembles the folded white-light system 120 of FIG. 7 but with the input and output fibers arranged in a same one- or two-dimensional array and with the mirror arrays 100, 106 integrated into a single one- or two-dimensional array.
A complex WDM or white-light network is subject to many problems. The different optical signals which are propagating on a particular link or being optically processed may have originated from different sources across the network. In a WDM system, the WDM wavelength output power may vary from transmitter to transmitter because of environmental changes, aging, or differences in power injected into the WDM stream. Different optical sources for either a WDM or white-light system are additionally subject to different amounts of attenuation over the extended network. Particularly, for a wavelength-routed transparent network, the WDM spectrum on a given fiber contains wavelength components which generally have traversed many diverse paths from different sources and with different losses and different impairment accumulation such as degradation of the optical signal-to-noise ratio or dispersion broadening. Further, wavelength multiplexing and demultiplexing usually rely on optical effects, such as diffraction or waveguide interference, which are very sensitive to absolute wavelength, which cannot be precisely controlled.
EDFAs or other optical amplifiers may be used to amplify optical signals to compensate loss, but they amplify the entire WDM signal and their gain spectrum is typically not flat. Therefore, measures are needed to maintain the power levels of different signals to be the same or at least in predetermined ratios.
In a complex WDM or white-light network, a signal may be switched multiple times. Each switching event needs to maximize transmission of the optical signal and minimize cross-talk between channels. A maximum of 10 dB attenuation through the switch and a minimum of 30 dB channel isolation are typical requirements. However, MEMS cross-connects and their associated optics are subject to internal variations of optical characteristics and misalignments, both integral to the device and as a result of both manufacturing and environmental variation and non-uniformity and of mechanical stress, all of which result in switch states having a significant variation and instability of insertion loss when aligned according to their nominal settings.
As described above, it is well known that WDM systems must maintain a significant degree of uniformity of power levels across the WDM spectrum, so that dynamic range considerations at receivers and amplifier, non-linear effects, and cross talk impairments can be minimized. As a result, serious attention must be payed to equalization of power levels across the spectrum. This equalization should be dynamic and under feedback control since the various wavelength components vary in intensity with time and due to changes in optical channel routing history among the components. One object of this invention is to provide means of equalization at multiple-fiber WDM switching nodes where many beams from diverse sources are interchanged among the fibers.
Bishop et al. have disclosed in U.S. Pat. No. 6,263,123 a pixellated WDM cross connect using a two-dimensional array of micromirrors in which a signal beam of a particular carrier wavelength is distributed to a plurality of the micromirrors. The number of mirrors reflecting light to an output port determines the transmission coefficient through the switch. When such a pixellated intensity control is used in a WDM cross-connect, the mirrors are arranged in a two-dimensional array and are tiltable about two axes. The described system is used primarily for characterizing the optical signal, not for controlling it. Similarly, Derickson et al. have disclosed in U.S. Pat. No. 4,796,479 a system for monitoring the intensities of the WDM channels in a WDM cross connect with the main emphasis on determining the ratio of signal to noise. It would be desirable to integrate the capability of such systems into an optical network without unduly increasing the cost and complexity.
An optical switching system includes a plurality of optical switching elements controllable in two different scales or dimensions depending upon the switch architecture and designed to be able to effect both switching and control of the transmission coefficient. The power adjustment is particularly useful in a wavelength-division multiplexing optical switch in which the power is adjusted between different wavelength channels combined in an output path such as an optical fiber.
An example of such a switching element is a mirror tiltable about two orthogonal directions or tiltable in one dimension according to a fine and a coarse resolution. In one embodiment, tilting about a major axis is used to control switching between optical ports, and tilting about the minor axis is used to control the amount of optical power passing through the switch.
Such mirrors are advantageously formed in a micro electromechanical system (MEMS) array. A mirror may be tilted by two pairs of electrostatic actuator electrodes positioned beneath the mirror on opposed sides of two torsion beams supporting the mirror.
A portion of the switched output signal may be diverted to an optical power monitor to enable feedback control of the power adjustment. Dynamic power equalization advantageously involves monitoring of power levels for the individual WDM wavelength channels on each fiber in the system. This information provides feedback for the power equalization mechanism. Input power may be advantageously also monitored for each wavelength channel.
The minor axis tilt may also be used for optimizing transmission through the optical switch. After the position of maximum transmission is established, the transmission may be detuned. An example of two-axis switching is the use of a major axis optimized for switching between fibers and a minor axis for adjusting transmission, especially insertion loss, of the same optical channel. Another example is the use of a coarse control of one or more major axes for establishing a switch state connection in combination with a fine control along one or more minor axes (which may or may not be the same as the major axes) to moderate the degree of coupling of a wavelength channel between the chosen fibers for that wavelength service.
The feedback control applies as well to white-light switching systems in which the same mirrors are used for switching and for transmission optimization and power equalization.
The minor axis tilt may be used to increase the high insertion loss for any optical connection in the off state and may further be used to turn the switch to a hard off during switching between discrete optical paths.